RFC: setjmp/longjmp (and friends) for a new port

In adding support for MIPS64 environments to NetBSD, a lot of the work
is just adapting the existing to nuances of new MIPS ABIs. Trying to
make minimal changes when possible is usually the best way to go. But
sometimes, a radical may be in order.

MIPS under NetBSD has always a jmp_buf that was a struct sigcontext.
Changing that for the existing code is almost impossible since you
have to live with the size limits of a jmp_buf. But a new port
doesn't have to worry about that, since there is no existing code to
be compatible with.

So I'm thinking that in MIPS64, setjmp/longjmp can just be versions of
getcontext/setcontext.
longjmp need to be able to set the return value, but that's a minor
variation.